Radiotelephones, which are well known in the art, generally refer to communications terminals which can provide a wireless communications link to one or more other communications terminals. Such radiotelephones are used in a variety of different applications, including terrestrial and satellite cellular telephone communications systems. In typical terrestrial cellular telephone systems, wireless transmissions from mobile users are received by local base stations or "cells" which retransmit the signal, via either a wireless link or the local telephone system, for reception by the intended receive terminals. In satellite "cellular" telephone systems, the satellite may either operate as the equivalent of a terrestrial local base station or, alternatively, may directly retransmit the signal to the intended receive terminal.
Many terrestrial cellular telephone systems rely primarily or exclusively on line-of-sight communications. In these systems numerous local cells are typically required to provide communications coverage for a large geographic area. The cost associated with providing such a large number of cells may prohibit the use of terrestrial cellular telephone systems in sparsely populated regions and/or areas where there is limited demand for cellular service. Moreover, even in areas where terrestrial cellular service is not precluded by economic considerations, "blackout" areas often arise due to local terrain and weather conditions.
In light of the above limitations with terrestrial based cellular telephone systems, combined terrestrial satellite communications networks have been proposed for providing cellular telephone service in regions which are not well suited for traditional terrestrial cellular systems. In these proposed systems, a limited terrestrial based cellular network is supplemented by a satellite communications network to provide communications for mobile users over a large geographical area. The terrestrial based cellular stations could thus be provided in high traffic areas, while a satellite communications network would provide service to remaining areas. In order to provide both cellular and satellite communications, the radiotelephones used with this system would typically include two transceivers, one for communicating with the terrestrial network and a second for communicating with the satellite. These combined cellular/satellite communications systems could provide full communications coverage over a wide geographic area without requiring an excessive number of terrestrial cells.
One such proposed terrestrial satellite cellular communications system is the Asian Cellular Satellite System. In this system, the satellite network will be implemented as one or more geosynchronous satellites orbiting approximately 22,600 miles above the equator that provide spot beam coverage over much of the far east, including China, Japan, Indonesia and the Philippines. In this system, signals transmitted to the satellite will fall within the 1626.5 MHz to 1660.5 MHz transmit frequency band, and signals transmitted from the satellite will fall within the 1525 MHz to 1559 MHz receive frequency band. Terrestrial cellular communications may then be implemented as a standard AMPS network, which operates in the 824 MHz to 894 MHz frequency band, or as a GSM network which operates in the 890 MHz to 960 MHz frequency band.
While integrating satellite and cellular service together in a dual-mode system may overcome many of the disadvantages associated with exclusively terrestrial based cellular systems, providing dual-mode radiotelephones that meet consumer size, weight, cost, and performance expectations is a significant challenge. These consumer expectations have been defined by the radiotelephones used with conventional terrestrial cellular systems, which only include a single transceiver which is designed to communicate with a cellular node which typically is located less than 20 miles from the mobile user. By way of contrast, the handheld radiotelephones which will be used with the Asian Cellular Satellite System must include both a terrestrial cellular and a satellite transceiver. Moreover, the large free space loss associated with the satellite communications aspect of the system may significantly increase the power and antenna gain which must be provided by the antenna for the satellite transceiver on the radiotelephone, as the signals transmitted to or from the satellites undergo a high degree of attenuation in traveling the 25,000 or more miles that typically separates the radiotelephone from the geosynchronous satellites.
Furthermore, the satellite aspects of the network also may impose additional constraints on the handheld user radiotelephone. For instance, the satellite transceiver on the radiotelephone preferably should provide a quasi-hemispherical antenna radiation pattern (in order to avoid the need to track a desired satellite) as opposed to the doughnut-shaped radiation pattern which is typically preferred for terrestrial cellular applications. Additionally, when communicating with the satellite, the radiotelephone should transmit and receive a circularly polarized waveform, so as to minimize the signal loss resulting from misalignment of the satellite and radiotelephone antennas and to avoid the effects of Faraday rotation which may result when the signal passes through the ionosphere. Conversely, when communication with terrestrial base stations, the radiotelephone will typically need to operate with a linear polarization.
In light of the above constraints, there is a need for handheld radiotelephones, and more specifically, antenna systems for such radiotelephones, which are capable of meeting the dual radiation pattern, operating frequency and polarization requirements mandated by combined terrestrial/satellite cellular communications networks. Moreover, given the handheld nature of the user terminals and consumer expectations of an antenna which is conveniently small for ease of portability, the antenna system capable of meeting the aforementioned requirements should fit within a small physical volume.